Jones—Relation of Soil Temperature to Disease in Plants. 443 
mental variation, soil temperature is so dominant a factor as to 
deserve the first consideration that we have given it in the present 
investigations. Work with the other variable factors should then 
follow in natural sequence, and finally some correlations should be 
attempted. 
Some Typical Results 
It must suffice now to select for summary the results from three 
such types as will best show the general nature of the problems 
met and the evidence secured in these studies concerning the rela¬ 
tion of soil temperature to disease development. For the present 
purposes it seems best to discuss a typical disease on each of three 
host plants belonging to one family, the Solanaceae. Two of these, 
the tomato and the potato, are so closely related botanically that the 
one may be grafted upon the other, yet they differ markedly in 
their temperature relations, the potato being favored by a cool 
climate and the tomato thriving best at higher temperatures. The 
tobacco, which is the third host selected, is also favored by rela¬ 
tively high temperatures. All of these naturally show great vege¬ 
tative vigor when free from disease, but all are subject to the at¬ 
tack of one or another subterranean parasite capable of rapidly 
causing serious disease or death. The parasites selected represent 
quite different temperature relations and modes of attack. It will 
be best, therefore, to consider each of these diseases separately and 
then to make certain comparisons. 
The Fusarium Wilt Disease of the Tomato 
This disease is caused by Fusarium lycopersici, of the mycological 
group known as the Imperfect Fungi. It is widespread throughout 
the warmer tomato-growing regions of the United States, but 
causes little or no trouble in the northern or cooler districts of 
commercial tomato culture. Thus, in the eastern Mississippi Val¬ 
ley states it is highly destructive from Louisiana north through 
Illinois and Indiana, but is so rarely observed that it has not be¬ 
come a recognizable factor in commercial tomato culture even on 
the relatively warm trucking soils where tomatoes are so much 
grown in southeastern and southwestern Wisconsin. These ob¬ 
servations, together with the discovery early in this work that the 
like Fusarium disease of both cabbage and flax is limited by soil 
